In semiconductor technologies, a semiconductor wafer is processed through various fabrication steps to form integrated circuits defined in multiple chips. Those fabrication steps include lithography patterning, etching, ion implantation, deposition, oxidation and thermal annealing. Different tools are used to perform these different types of processes on semiconductor wafers. Thus, the wafers must be transferred between tools after each process is performed.
It is generally desirable to avoid human contact with the semiconductor wafers as they are transferred because various contaminants or static discharges may harm the wafers. Thus, wafer transportation pods such as front opening universal pods (FOUPs), which provide a controlled environment, are used to move the semiconductor wafers throughout the fabrication facility to different tools. Robotic mechanisms are used to remove semiconductor wafers and insert them into the fabrication tools.
In some cases, the FOUPs are transported through the fabrication facility by an overhead hoist transfer (OHT) system. The OHT includes several tracks above the ground. A carrier device moves along the tracks and can be lowered to pick up or drop off FOUPs as needed. In order to improve the efficiency of semiconductor fabrication, it is desirable to find ways to increase the efficiency of FOUP transportation.